This invention relates generally to the field of telephony, and more particularly to an improved form of secondary or backup protection for individual subscriber pair protector modules normally installed upon a mainframe in a telephone office.
Such protector modules traditionally employ a pair of carbon electrodes which arc upon the occurrence of momentary current overload to ground the individual circuit. Later developments include heat-sensitive devices employing a fusible solder member which, upon fusing, allows a resilient member to permanently short the module to ground.
A still later development, now in wide spread use is the so-called three element gas tube in which momentary overloads cause the tubes to become conductive to short the overload to ground, and in which, upon the occurrence of a sustained overload, the tube develops sufficient heat to activate a separate heat-sensitive device to cause permanent shorting to ground.
With the development of requirements for protective modules of ever smaller dimensions consistent with connector blocks having ever higher circuit densities, the use of conventional heat sensitive devices including heat coils becomes more difficult because of space limitations, and it has become necessary to provide a heat-sensitive element of sufficiently simple construction to supplement the action of the three-element gas tube without requiring significant additional volume within the protector module housing. Further, in some cases, there is a requirement for secondary air gap protection which will provide protection against momentary overloads in the event of a defective gas tube in which the conductivity voltage levels are other than standard, apart from the function of the heat-sensitive element.